1. Field of the Invention
The present invention relates to a fuel cell which is capable of controlling the flow rates of reactant gases and a coolant, and a method of controlling such a fuel cell.
2. Description of the Related Art
Generally, solid polymer electrolyte fuel cells include an electrolyte membrane comprising a polymer ion exchange membrane. Specifically, the solid polymer electrolyte fuel, cell has a unit cell (electric power generating unit cell) comprising a membrane electrode assembly (MEA) including an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode. Each of the anode and the cathode comprises a base primarily made of carbon and an electrode catalyst layer of a precious metal joined to the base. The membrane electrode assembly is interposed between separators (bipolar plates) each including a coolant passage. Typically, the solid polymer electrolyte fuel cell employs a plurality of such unit cells that are stacked into a cell assembly.
In the cell assembly, a fuel gas such as a hydrogen-containing gas is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electric current. An oxygen-containing gas or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
For use on vehicles, the cell assembly is required to be able to produce a relatively large output level and also to be small. To meet this requirement, there has been proposed a structure comprising a plurality of relatively compact unit cells that are stacked together.
While in operation, the cell assembly is subject to different conditions such as under a low load at a low temperature, under a high load at a high temperature, etc. A unit cell in the cell assembly which serves as an inlet for reactant gases (a fuel gas and an oxygen-containing gas) and another unit cell in the cell assembly which serves as an outlet for reactant gases occasionally fail to be optimized to such different operating conditions of the cell assembly. Specifically, a temperature distribution is likely to be developed along the stack of unit cells, and the relative humidities in the fuel gas and the oxygen-containing gas tend to vary. If the electrolyte membrane is dried due to a temperature rise caused by such a temperature distribution and variations of the relative humidities, then hydrogen ions in the electrolyte membrane are impeded against movement and hence provides a resistance, lowering the ability of the cell assembly to generate electric power. If the relative humidity in the oxygen-containing gas rises, then the cathode generates excessive water due to the relative humidity and water generated in the electrochemical reaction, causing water condensation and water clogging in the gas passage which obstructs the flow of the oxygen-containing gas. As a result, the cathode becomes less reactive, also lowering the ability of the cell assembly to generate electric power.